WordPress offers Essential Grid 4 under Dashboard › Updates and on the Plugins page. Is your copy not registered, for example because it came with your theme? Register it for free first, or update it by hand. Your sites, grids and settings stay as they are.
With a CodeCanyon or one-time licence, the premium add-ons and templates already installed on your sites keep working; new premium downloads need a yearly licence. The upgrade page has the steps for every case.